it makes no sense to continue with an undefined vmid.

Fixes: c8b0507f ("drm/amdkfd: judge get_atc_vmid_pasid_mapping_info before call")
Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>